1. Field of the Invention
This invention relates to the field of wireless communications, and more particularly to the determination of decision thresholds for FSK demodulators.
2. Description of the Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter of a transceiver includes a data modulation stage, one or more intermediate frequency (IF) stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more IF stages mix the baseband signals with the signal generated by one or more local oscillators to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more IF stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more IF stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signals into baseband signals or IF signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
One of the modulation/demodulation techniques employed by such wireless communication standards is known as FSK (frequency shift-key) modulation. For FSK modulation, the modulation stage of the transceiver modulates binary information onto an analog carrier signal having a frequency fC. The carrier signal is increased in frequency to a signal f1=fC+fΔ, which represents one of the binary values (e.g., a binary 1) and is decreased in frequency to a second frequency f0=fC−fΔwhich represents the opposite binary state (e.g., a binary zero). FIGS. 1 and 2 illustrate this concept. Carrier signal 100 is modulated such that an increase in frequency is a binary 1 and a decrease in frequency is a binary 0, as shown. The digital information, which is often grouped together for transmission as packets of the digital data, is serially converted by the transceiver's modulation stage into the FSK modulated analog signal, and then transmitted, or broadcasted, to one or more other transceivers in the network.
The receiving transceiver(s) typically includes a discriminator as part of the demodulation stage that is capable of detecting (or discriminating between) the two frequencies as the signal is received, and producing an output voltage that is directly related to the frequency of the received signal. This is sometimes known as a frequency-to-voltage conversion. Typically, all of the transceivers for a network are designed to operate about the predetermined center or carrier frequency of the modulation stages. Thus, the output of the discriminator ideally produces a voltage V1=VCVΔ for a binary one and a voltage V0=VC+VΔ for binary zero, where VC is a voltage representative of fC and VΔ is a voltage that is representative of fΔ. The output of such a discriminator is illustrated in FIG. 3.
Because the center frequency of each transceiver typically has a tolerance of as much as ±150 kHz of the expected frequency, however, a DC offset voltage is produced during the frequency voltage conversion that is linearly related to the frequency offset. Thus, the value of the output of the voltage-to-frequency converter may be offset by a voltage such that V1=VC+VΔ=2VΔ. In such a case, the ideal center point VC of FIG. 3 will no longer be the ideal comparison or decision point for determining whether the output is representing a binary zero or a one.
One common method of determining the center or slicing point of a discriminator output 102 is through use of a peak and valley detector. FIG. 4 illustrates the imposition of an offset voltage VOFF on discriminator output 102 that forces a peak and valley detector to start at an extreme position. Thus, for the data to be correctly demodulated in light of this potentially time varying offset, the analog-to-digital converter (ADC) that converts the raw output of the frequency to analog converter must be able to dynamically locate an appropriate center point 550 above which is a binary 1, and below which is a binary 0. The peak and valley detector produces an output that quickly attacks (i.e., tracks) the discriminator output in the positive direction and stores a peak value 502 of the voltage for any given point in time. That peak value is then permitted to decay until another positive-going signal of the discriminator output exceeds the decaying value, at which point the greater voltage is stored.
Likewise, the peak and valley detector does the same and produces a decaying peak value 504 from an initial offset value and a decaying peak value 506 from subsequent negative peaks (valleys). The slice point is then along the line 506, and is dynamically determined to be the halfway point of the difference between the current values of the peak and valley detectors (i.e., Vp−Vv). The decay rate of the two detectors should be such that they will detect peaks and valleys that are less than the peaks or valleys previously detected (i.e., when the offset changes). As is illustrated by FIG. 4, some, if not all, of the first three bits will go undetected given this decay rate because the slice point along the line 506 does not reach an ideal location for distinguishing between levels until approximately point 508.
On the other hand, one does not want the decay rate too fast, even though that may improve the data acquisition time. If too many of the same bit values are transmitted sequentially, and the decay rate is too fast, the detector output that is decaying will rapidly approach the other detector's value until they are virtually equal. FIG. 5 illustrates this scenario. Line 404 illustrates the decay of the valley detector as it spans several binary ones in the signal. As can be seen from FIG. 5, if the same bit state is present long enough, the decay of the valley detector will eventually bring it very close to the value of the positive voltage swing at V1=VC+VΔ. If this occurs, it will be clear to those of average skill in the art that this reduces the noise margin so severely for the ADC that even the slightest bit of noise will cause the ADC to toggle based on noise present in discriminator output 102. This in turn increases the Bit Error Rate of the channel significantly.
Prior art solutions have typically constrained the decay rate based on the known maximum number of the same bits that will be received in a row, and to ensure that if such a transmission is received, the slicing point is always within a range that provides ample noise margin until the next bit toggle comes along. This solution has heretofore been a reasonably acceptable one because applications of FSK have been primarily for lower rate transmission standards, or ones that have sufficiently long headers that provide ample time to capture the signal, even when the peak detectors have somewhat slow decay rates.
One example of a specific wireless network standard is one based on the Bluetooth standard, which is designed to facilitate short-range (i.e., 30 to 60 feet) wireless communication between terminal equipment, such as PC's, laptops, printers, faxes, and hand-held devices, such as PDAs (personal digital assistants). The Bluetooth standard defines a standard by which devices, such as the foregoing, transmit and receive signals using the ISM (industrial, scientific and medical) radio band of 2.4 GHz. This standard has been established to promote the networking of such devices through compatible transceivers so that they may communicate with one another without need for physical interconnection through proprietary cables. The noise and signal strength issues for a Bluetooth wireless network are analogous to the cellular telephone network, albeit over much shorter distances.
One of the characteristics of Bluetooth is its relatively high transmission frequency of 2.4 GHz and its extremely short preamble. The preamble is typically only 4 bits toggling between 0 and 1. Thus, in view of a large offset embedded within an incoming transmission, such as that illustrated in FIG. 4, a peak or valley detector designed to slowly take so long to decay that the peaks or valleys of several bits beyond the preamble could be missed. If the decay rate is too fast, there may be situations where the peak and valley outputs become too close to another to provide an appropriate slicing point.
Therefore, there is a need in the art for an FSK slice point determination method and apparatus to permit fast acquisition of packet preamble information, while ensuring that noise margin for the ADC is maintained thereby keeping the Bit Error Rate for the system low.